Many types of monomers undergo shrinkage during polymerization to a degree that makes them generally unsuited for use in numerous applications, including for use as stress-free composites, high-strength adhesives, and precision castings. As an example, when such monomers are used in composites which contain inorganic fillers, the polymeric matrix is subject to failure when the polymer shrinks and pulls away from the filler particles. Failure of the composite can also occur when the matrix ruptures as a result of voids or micro cracks which form in the matrix during polymerization shrinkage.
Polymeric matrices commonly employed in dental materials such as adhesives and composites are based on 2,2′-bis[4-(2-hydroxy-3-methacryloyloxypropoxy)]phenyl propane (BisGMA). A significant problem associated with the use of this monomer in dental applications is the shrinkage which occurs as the monomer is polymerized. The BisGMA monomer itself typically experiences high shrinkage, and when a low viscosity reactive diluent is combined with the monomer, the shrinkage may even be higher. The adverse effects of such shrinkage are believed to include increased postoperative sensitivity, the formation of marginal gaps between the dental restoration and the cavity wall, cracking of the restoration, and microleakage and potential failure of the restoration.
The discovery that spiroorthocarbonates may undergo reduced polymerization contraction and possibly polymerization expansion has led to the suggestion of their use in reinforced composites, including as dental materials. Spiroorthocarbonates are esters of orthocarboxylic acid and have four oxygen atoms bonded to a single carbon atom, with the carbon atom being common to two ring systems. The expansion of the spiroorthocarbonates on polymerization is attributed to a double spiro-cyclic ring opening of the spiroorthocarbonates, resulting in the breaking of two covalent bonds to form one new bond.
Initial attempts to form a homogeneous polymer matrix from certain spiroorthocarbonates and BisGMA resin mixtures proved unsuccessful because of the incomplete polymerization of the spiroorthocarbonates. Thompson et al., J. Dental Research 58:15221532 (1979). More recent studies demonstrated that homogeneous mixtures of other spiroorthocarbonates and BisGMA could be obtained. Stansbury, J. Dental Research 70:527; Abstract No. 2088 (1991).
The photocationic-initiated expansion polymerization of alicyclic spiroorthocarbonate monomers and the potential use of the resulting polymers in dental materials have been previously reported by some of the present inventors, with others. Byerley et al., Dent. Mater. 8:345-350 (1992). The specific spiroorthocarbonates identified by Byerley et al. include cis/cis, cis/trans, and trans/trans configurational isomers of 2,3,8,9-di(tetramethylene)-1,5,7,11-tetraoxaspiro[5.5]undecane. These spiroorthocarbonates were determined to undergo an expansion of 3.5% during homopolymerization and demonstrated acceptable cytotoxicity and genotoxicity properties, making them promising components of composite resin matrix materials.
Some of the present inventors, with others, have also previously reported on the preparation of a copolymer of an alicyclic spiroorthocarbonate and an unidentified monofunctional epoxide, with the observation that there were no indications of the formation of small ring compounds as polymerization by-products. Byerley et al., J. Dental Research 69:263; Abstract No. 1233 (1990). The copolymerization of trans/trans-2,3,8,9-di(tetramethylene)-1,5,7,11-tetraoxaspiro[5.5]undecane and commercially available multifunctional epoxides was also disclosed in a paper presented by Byerley et al., Abstract No. 1233, cited above. However, no physical or mechanical properties, including percentage shrinkage, of the copolymer compositions were disclosed. Still further, spiroorthocarbonate copolymers have been created that are capable of yielding a hard, non-shrinking matrix resin. These copolymers include a trans/trans-2,3,8,9-di(tetramethylene)-1,5,7,11-tetraoxaspiro[5.5]undecane spiroorthocarbonate, a polymerizable epoxy resin, and a hydroxyl containing material, as described in U.S. Pat. No. 5,808,108.
A diepoxy spiroorthocarbonate, namely, 3,23-dioxatrispiro[tricyclo[3.2.1.0<2,4>]octane-6,5′-1,3-dioxane-2′2″-1,3-dioxane-5″,7′″-tricyclo[3.2.1.0<2,4>octane], is disclosed in a book entitled, “Expanding Monomers, Synthesis, Characterization and Applications,” edited by R. J. Sadhir and R. M. Luck, CRC Press, Boca Raton (1992), pp. 329-332.
Some of the present inventors, with others, have previously disclosed the spiro-orthcarbonates 2,8-dimethyl-1,5,7,11-tetraoxaspiro[5,5]undecane and 3,9-diethyl-3,9-dipropionyloxymethyl-1,5,7,11-tetraoxaspiro[5,5]undecane in an article entitled “Photoreactivity of Expanding Monomers and Epoxy-Based Matrix Resin Systems” by Chappelow et al., J. of Applied Polymer Science, Vol 76, 1715-1724 (2000).
However, none of the above disclosed spiro-orthocarbonates used in a photopolymerizable composition generally exhibit characteristics desirable in a dental restorative, including flexible linkages, enhanced miscibility and reactivity and reduced photopolymerization contraction stress.
Despite the advances resulting from the above-noted polymeric compositions and SOCs, a need still exists for cationically photopolymerizable compositions exhibiting properties such as reduced shrinkage and reduced photopolymerization contraction stress. Still further, there is a need for expanding monomers containing flexible linkages that provide enhanced miscibility and reactivity in photopolymerizable compositions.